The invention relates to a method for making possible greatest possible accelerations via frictional engagement and a method for determining the longitudinal force acting on a tire or the tire spring travel or the footprint length or the load/pressure ratio and a device for determining the longitudinal force acting on a tire or the tire spring travel or the footprint length or the load/pressure ratio during rotation and a vehicle tire that is suitable for cooperation with the aforementioned device and/or for the aforementioned method.
A vehicle wheel in the context of this application is to be understood as a combination of all components which, with the exception of small load-depending deformations, are torsion-proof connected to one another and are designed for rotation. A wheel thus includes especially the tire, the wheel rim with wheel flange and wheel rim bowl, the valve, the hub, possibly sealing and/or securing rings attached thereto, brake disks, anti-lock magnet wheels and optionally drive shafts.
The invention is intended to increase the safety level of motor vehicles on wheels that are provided with tires, especially pneumatic tires, that, at least in the longitudinal directions, in general also in the transverse direction (one exception is the famous Metro in Paris provided with pneumatic tires), can transmit forces onto the road etc. only by frictional engagement. In most operational states the maximum possible frictional engagement is not even used; however, wherever it is necessary to react to unpredictable events, for example, a vehicle ahead that has spun out of control or a child running onto the street, so as to prevent dangerous situations, the attainability of great accelerations is required, especially with negative sign, i.e., great braking forces.
It is known that the value of greatest possible acceleration depends substantially on the coefficient of friction between the tires and the street. It is furthermore known, that this coefficient of friction is affected by the paring of the material street/tire, mostly asphalt/rubber mixture, the air pressure, the footprint length, and also the tire tread profile and the weather conditions. Furthermore, it is known that the coefficient of friction is a function of slip. Slip is to be understood as the difference between circumferential velocity of the tire minus steering knuckle velocity, divided by the steering knuckle velocity.
FIG. 1 shows for the conventional frictional material pairing for typical boundary conditions a curve of the coefficient of friction .mu..sub.longitudinal as a function of slip, in the following referred to as slip curve. The maximum longitudinal coefficient of friction is reached at a slip of approximately 10%. When the slip is increased further, which could be achieved during braking by increasing the braking moment, the coefficient of friction, together with the effective longitudinal force, would not increase further but, to the contrary, would decrease. This not only would result in the problem that the braking deceleration would decrease instantly, but would also lead to, when maintaining the too high braking moment, the wheel rotation frequency and thus the circumferential velocity of the tire would be reduced quickly to zero (the quicker, the smaller the moment of inertia of the tire--and it is relatively small in comparison to the vehicle mass). The operational state in which the wheel no longer rotates despite still present steering knuckle velocity, is called "locking". The slip is then -100%.
FIG. 2 shows in a solid line the slip curve for the same tire on cold ice (more unfavorable for warmer ice), and, as a comparison, in a thin dashed line the slip curve of FIG. 1 is shown again. The value .mu..sub.max is not only substantially lower but also occurs at smaller slip.
The runaway rotational deceleration of the tire occurring during vehicle deceleration, already at slightly increased braking moment, enhances the drawback of the initially only somewhat too great braking slip: It increases quickly from, for example, -14% to -100% in its value. Due to this effect of surpassing the slip, to which the maximum coefficient of friction is related, the area past this slip is often designated by persons skilled in the art as instable slip area. The slip area between 0.degree. and this value is designated as stable. The slip to which the maximum coefficient of friction is related is called critical slip.
The same fact holds true for drive slip that is too great. Spinning drive wheels also effect negatively the safety of the vehicle, even though usually not as badly as braked wheels that lock. Furthermore, in the conventional non-locking differentials, the drive force does not break down, as during braking, for each wheel but for the axle because the greater portion of the drive output is transmitted to the slipping wheel. For non-locking interaxle differentials the drive force would even break down almost completely.
In addition to the decrease of the transmittable longitudinal force, for braking with locking as well as for slipping drive wheels, the vehicle safety in such operational states is impeded by the loss, in the case of locking brakes the complete loss of the ability to transmit lateral forces. The straight running stability is thus only supported by the translatory inertia mass and the moment of inertia mass about the vertical vehicle axis; steering maneuvers are impossible.
Because of the great importance of adjusting the correct slip for the requirement of greatest possible positive or negative acceleration and because of the fact that a human being as a controller is usually only capable of simultaneously maintaining a maximum of two wheels within the optimal slip range, as in the case of a motorcycle, whereby for all other motor vehicles including airplanes, in general, only one actuating device for the entire number of wheel brakes is present, the development of slip control systems, i.e., of systems where a technical device takes over the control function performed by the human being began in the forties of this century, initially only for the braking systems of airplane landing gear. When using such systems, the human being by adjusting the lever pressure, lever travel or pedal pressure or pedal travel etc. only transmits his desire for controlling the nominal value, for example, the brake acceleration.
The control system, on the other hand, has been assigned the tank to adjust for each wheel individually the favorable slip. Most of the slip control systems will only function when at one wheel almost the critical slip has been reached. By preventing a further increase of the brake, respectively, drive moment, locking, respectively, slipping is prevented. Once the critical slip has been surpassed, for example, when the wheel suddenly encounters a worse frictional pairing as for example blue basalt etc., the control system reduces the brake, respectively, drive moment to such an extent and for such a period time until the slip has been adjusted to just below the critical slip.
Slip control devices have been used for approximately 8 years in the mass production of passenger cars, trucks and trailers with increasing market share. Insofar as they control only the brake slip, the acronym ABS (derived from Anti Blocking System) has been used. Slip control systems prove their effectiveness especially impressively under such driving conditions where one wheel track runs on a surface with bad maximum coefficient of friction while the other wheel track runs on a surface with high maximum coefficient of friction.
Slip control systems according to the prior art detect very precisely the actual rpm of each wheel. For this purpose, each wheel is provided with a so-called magnet wheel that on a circumferential line comprises a plurality of marks, the passing of which is detected by a non-rotatingly arranged sensor based on fluctuations of the magnetic flux. From the time interval between passing of two adjacently arranged marks of the magnet wheel, the microcomputer of the control device calculates the wheel rpm and, after multiplication with a stored circumferential length, the circumferential velocity of the tire.
Based on this data the electronic control device calculates furthermore the change of rpm or the circumferential velocity over time.
For detecting the actual slip, each slip control system requires information in regard to the steering knuckle velocity. Since in most operational states of interest the velocity differences between the steering knuckle (curve-inner steering knuckles somewhat slower than curve-outer steering knuckles) are minimal, the slip control systems known to the inventors therefore set all steering knuckle velocities to be equal to the (translatory) vehicle velocity. However, there remains the problem how to determine exactly this vehicle velocity.
For this purpose, the control device also determines, based on the rpm or circumferential velocities of a plurality of wheels at the vehicle, in general, of all wheels, the maximum (during braking), respectively, the minimum (during positive acceleration). Even though out of all wheel rotation information these extreme velocities, respectively, extreme rpm in reality correlate best with the vehicle velocity, but, it must still be considered fictitious as long as it is not measured slip-free, i.e., free of braking and drive moments. Based on this possibly fictitious vehicle velocity, the control device calculates the slip individually for each wheel based on the wheel rotation information of the individual wheels.
When a vehicle over a longer period of time is braked with slip at all four wheels, the possible deviation between the actual vehicle velocity and the aforementioned fictitiously calculated one increases steadily so that the information basis is more and more dubious. When, however, the vehicle velocity can no longer be determined reliably, the system loses the required information for reliable operation in regard to the individual wheel slip, and the quality of the control decreases. This problem exists for drive slip control systems as well as for brake slip control systems when all axles are driven.
These problems can be overcome when in sufficiently short time intervals at least one wheel is made substantially free of moments, and the circumferential velocity of this wheel thus approaches the vehicle velocity (intermittent braking). The time interval of making the wheel moment-free can be shorter when the moment of inertia of the respective wheel is minimal; however, the moments of inertia of wheels in the passenger vehicle field have not been returned due to wider wheels and rims, stronger brakes, and stronger drive joints despite an increasing use of light metal for the wheel rings. Unavoidably, when freeing the wheel of moments, the braking and acceleration capability is wasted.
In addition to the aforementioned comparison of the rotational velocities, it is also known for detecting over-critical slip at a wheel additionally or alternatively, to use the comparison of the rotational accelerations. When at one wheel the value of rotational acceleration surpasses the value of rotational acceleration at the other wheels, this is interpreted as the beginning of locking, respectively, slipping and the braking, respectively, drive moment of this wheel is controlled to a smaller value.
But even with this measuring method problems result when, for example, during the sudden occurrence of an oil stain, all wheels encounter a rotational acceleration that is too great for the instant coefficient of friction. When the rotational accelerations have not yet increased to a value that should not even be reached for .mu.=1, even an additional program loop with a plausibility control does not help.
It is furthermore known that for some roadway coatings such as ice, snow or gravel up to this time no sufficient slip control has been possible. The slip curves for these conditions deviate greatly from the slip curve for rubber/asphalt represented in FIG. 1. For a more exact understanding of the instantly applicable slip curve the control process could be improved. This would entail the recognition of the roadway coating and an adaptation of the control behavior of the slip control device to the roadway coating.
In summary and abstraction, a problem of the known slip control system is that it employs data for controlling the slip which have been themselves measured under slip conditions. It is a first object of the invention to at least reduce the dependency on slip-dependently measured values, respectively, to preferably overcome it.
Most vehicles driven by frictional engagement use tires which obtain their supporting capacity and their optimal frictional engagement values only by filling with compressed air. When the tire pressure drops below a threshold value depending on the wheel load, the safety is negatively affected by this also. Therefore, many systems for controlling the air pressure have been suggested. They all have the problem that the value to be measured, i.e., the air pressure, is present within the rotating wheel, but the measured value is to be used in a non-rotating system, for example, to be displayed at the dashboard.
Accordingly, all air pressure controlling devices can be divided into two main groups:
In a first group all suggestions are to be arranged where the interior of the tire opens via a channel, penetrating the wheel rim and the hub, with a sliding sealing into the non-rotating steering knuckle and from there opens via a hose etc. into a non-rotating pressure meter. Such systems allow, in addition to the actual measuring, also the correction of a possibly recognized error. A non-rotatingly arranged compressor is able to supply compressed air into the interior of the tire in the reverse direction. The disadvantage of all systems of this group is the limited service life of the pressurized sliding seals and the relatively great leakage flow which not only allows for a compressor but almost makes it necessary to have one for most applications. However, this results in additional weight, increased energy consumption, and a considerably higher purchase price.
In the second group all such suggestions are arranged where the pressure meter is arranged in the rotating wheel and the measured data are supplied to a non-rotating computing unit. This data transmission can be performed with slip rings or by radio transmission. In any case, the expenditure for such an arrangement is great. Slip rings increase the frictional resistance and are subject to wear reducing the service life, radio transmitters require an energy supply into the rotating wheel or an energy source within the rotating wheel, for example, a battery.
It is thus a second object of the invention to monitor the presence of sufficient air pressure in a simple and reliable manner.